The present invention relates to keyswitches for use in computer terminals, typewriters, and the like and more particularly to an actuator assembly for use in a linear feel keyswitch with hysteresis which includes three separate spring elements.
Keyswitches of conventional design generally include a plunger which is slideably mounted within a housing. The plunger is designed to slide downwardly when the keytop of the keyswitch is pressed. Downward pressure on the keytop actuates an electrical switch coupled to the plunger.
For certain applications, it is desirable to have a keyswitch which provides a linear feel to the keyswitch operator. It is also desirable to provide a keyswitch with hysteresis and overtravel.
In a keyswitch with linear feel, the amount of force required to press the switch from its non-actuated to its fully depressed condition increases linearly as the plunger travels downwardly. In a keyboard, such as a typewriter keyboard manufactured from a plurality of keyswitches, the provision of linear feel reduces fatigue on the fingers of the keyboard operator. The use of linear feel keyswitches also increases accuracy by providing a certain degree of feedback to the keyboard operator and facilitates the efficiency of keyboard operation.
The provision of hysteresis in a keyswitch results in a lag in the electrical response of the switch. For example, if the point at which a keyswitch "makes" an electrical connection on the downward stroke of the plunger is further down than the point at which the electrical connection "breaks" on the upward stroke, the switch is said to have a positive hysteresis. If the make and break points in the switch are at the exact same physical location on the downward and upward strokes (i.e., no hysteresis), it will be possible for a keyswitch operator to unintentionally produce multiple actuations if any hesitation is made at the exact moment the switch turns on. This phenomenon is known as "teasing" the switch, because what is intended by the operator as one switch actuation may be interpreted by the circuitry actuated by the switch as a plurality of actuations. Where the switch is designed such that it must pass through the make point on its return stroke before the break point is reached, such teasing, with the resultant possibility of error, will be prevented.
Hysteresis can be provided by either electrical means, mechanical means, or both. For example, hysteresis can be provided electrically in a capacitive-type keyboard by using an electronic circuit with different thresholds for "make" and "break" to detect the keyboard output. If the electronic circuit senses an incremental change in capacitance of 8 picofarads ("pf") from a threshold value as a make condition and an incremental capacitance change of 5 pf from the same threshold value as a break condition, then the plungers of the keyswitches on the keyboard will have to pass through the make point on their return stroke before reaching the break point. Thus, positive hysteresis will be achieved.
Mechanical hysteresis can be provided by mechanically establishing a different plunger displacement for the make point and the break point. It has now been found that the use of a plunger using three spring elements, where one spring element is a resilient foam switch actuator, provides a significantly improved positive mechanical hysteresis. This result occurs even though the resilient foam itself has a force versus displacement profile indicative of a negative hysteresis. Spring elements other than resilient foam, which exhibit negative hysteresis, can alternatively be used. For example, a plunger having two spring elements, in conjunction with the spring force provided by a momentary contact membrane-type keyswitch well known in the art, can provide an improved positive mechanical hysteresis in accordance with the present invention. In such an arrangement, the plunger with two spring elements would be used to actuate the membrane-type switch.
In a keyswitch with overtravel, electrical contact is made (i.e., the switch turns "on") before the keytop is fully depressed. Overtravel provides for more reliable switch operation, because keyboard operators often inadvertently fail to press the keyswitch down the whole way, particularly when typing at a high rate of speed. When overtravel is provided, the keyswitch will turn on as long as the plunger is depressed at least to the point where electrical contact is made.
It would be advantageous to provide a keyswitch combining the favorable characteristics of linear force, hysteresis, and overtravel. The present invention relates to such a keyswitch.